From my own experience, I would disagree.
When you get hot, you sweat a lot, and you need to replace these salts. If you just drink loads of water without replacing salts, its not enough. I found that out the hard way in the Cantabrian mountains many years ago. I'd drunk lots of water but still felt dreadful, nauseous, vomited, went cold and felt disorientated. I later read that too much water can be as dangerous as too little sometimes.
When my children were small, I was told by my doctor, that a good cheap rehydration fluid could be made by adding a pinch of table salt and a teaspoon of sugar to a glass of water. It definitely does not contain all the salts you need to replace, but it leaves you feeling so much better that you can then eat a proper meal and get the other salts you need from your food.
We used to walk in the mountains of Oman in the Middle East quite frequently, with temps in the mid to high 30s in the afternoon. For lunch, we always started with a small bottle of very sweet fruit juice, followed by a bag of crisps. Never got dehydrated on any of these walks. A couple of weeks ago, I walked 32 km (more than I had intended) to Cee. It was warm but overcast to begin with, then the sun came out later. I had a bag of unsalted nuts and raisins for lunch, along with some chocolate. On arrival in Cee, I started to feel nauseous and cold. I lay on my bunk for a short while, feeling miserable. Thankfully, I suddenly realised that I was dehydrated. A mug of water+salt+sugar tasted wonderful, so I had a second and within 10 mins I was fully recovered.
